Toggling data usage for most and/or individual apps?

Hello, I'm traveling out of the country soon and it's going to be a major change for me because I'll be going from an unlimited data plan to a very restricted amount of data. I have a lot of background processes, push notifications, syncing, etc going, which normally consumes a lot amount of data.

When I'm out of country, I want to essentially stop all data and syncing save for whatever VoIP and messaging solutions I choose.

So for example, I want to limit Gmail, Facebook, Instagram, Snapchat, Google app, and other the other stuff that's constantly using data, pushing notifications, etc, but still have messages/calls come through on Hangouts, Google Voice, Skype, WhatsApp, Facebook messenger, or whatever I decide to use. Ideally I'd like to download Google or Waze maps on WiFi (I assume both apps cover Italy) and then turn off data and only use the GPS radio. Still, if I need to turn data back on for Google Maps, or to check my email, I don't necessarily want data and syncing turning back on for all other apps, and I don't want the app to continue using data when I'm done with it.

I also want to ability to remove the limits on all the apps as soon as I'm in WiFi. However, WiFi can be unreliable, so I don't want to enable data for everything and accidentally have the WiFi drop and the 3G/4G turn on. I assume this means turning off the radios, however, I don't want to lose the ability to text/call (emergencies only) so I'd want to leave that particular radio on.

Is there an app or Android feature that would let me play with data usage in this manner? I'm rooted running stock 6.0.1 on my Nexus 6P.

Thanks!

PS. Unless Tasker is a lot more fool proof and easy to learn/setup than it used to be, I just can't use that as my solution right now due to lack of time before I leave.

Thanks Tim. All of what you said is absolutely part of my plan, but I would like to be able to turn on data while I'm there using the prepaid sim's data without having to worry about my apps just all doing a data grab, which is exactly what happens if I just leave things set up the way they are currently.

To complicate the matter further, I'm actually going to need to keep roaming enabled, I think. I'm going to be buying a SIM card from the Netherlands and they roam for free in most countries (I'll be in Italy) so it won't rack up additional charges.

I guess my search for how to limit data usage without turning it off continues.